INTRODUCTION: In agricultural production, the utilization of heterosis has brought significant benefits to society and economic development by markedly increasing crop yield, stress resistance, and quality. Foxtail millet (Setaria italica var. germanica), as an important coarse grain crop in the arid and semi-arid regions of northern China, holds a significant position in dry land ecological agriculture. However, the slow increase in the yield of foxtail millet has limited the further realization of its production potential. Utilizing heterosis has thus become one of the effective ways to increase the yield of foxtail millet. Nevertheless, research on the physiological and molecular mechanisms of its heterosis is still relatively weak, and the mechanism remains unclear. Therefore, to understand the physiological mechanisms of heterosis in foxtail millet is of great importance for improving the yield of hybrid varieties.
RATIONALE: The Changzagu series of foxtail millet hybrids (Changzagu466, Changzagu2922, and Changzagu333) exhibit significant heterosis in yield. In order to elucidate its mechanism, we systematically analyzed the yield advantages of these hybrids and their influencing factors by measuring yield-related traits and key physiological indicators of the hybrids and their parental lines.
RESULTS: Throughout the entire growth period, the chlorophyll content of the three hybrid varieties was higher than that of their parents. Among them, Changzagu466 exhibited the highest chlorophyll content at the jointing stage, reaching 13.86 mg∙g^-1 FW. During the seedling and jointing stages, the root activity of the three hybrids was significantly higher than that of their parents. Specifically, Changzagu466 showed the highest root activity at the seedling stage, measuring 1.76 mg∙g^-1∙h^-1, which was 7.8 times and 5.5 times higher than its female and male parents, respectively. The root activity values of Changzagu2922 at the seedling stage were 0.38 and 0.66 mg∙g^-1∙h^-1 higher than its female and male parents, respectively. Meanwhile, Changzagu333 displayed pronounced advantages at the jointing stage, with root activity values 0.31 and 0.62 mg∙g^-1∙h^-1 higher than its female and male parents, respectively. In terms of yield-related traits, compared to their parents, the hybrids showed significant improvements in both grain filling rate and spikelet number. Changzagu466 reached its maximum grain filling rate of 1.58 g∙d^-1 per panicle at 19 days after flowering. Both Changzagu466 and Changzagu333 had significantly higher spikelet numbers than their parents, while Changzagu2922 also showed a significant increase in spikelet numbers. In addition, the hybrid varieties also demonstrate certain advantages in root nitrogen accumulation and nitrogen translocation efficiency. Among them, Changzagu2922 exhibits the strongest root nitrogen accumulation advantage and the highest nitrogen translocation efficiency (nearly 56%), both are significantly higher than that of its male parent line M22.
CONCLUSION: The Changzagu series of foxtail millet hybrids effectively enhances photosynthetic capacity, nutrient absorption and utilization efficiency by significantly increasing chlorophyll content, root activity during the early growth stages, and root nitrogen accumulation. Meanwhile, the significant increase in grain filling rate and spikelet number of the hybrids further enhances both grain weight and grain number per panicle, ultimately achieving high yield.

Foxtail millet and parental trait comparison chart